ss: fix NULL pointer access when parsing unix sockets with oldformat

ss: fix NULL pointer access when parsing unix sockets with oldformat

When parsing and printing the unix sockets in unix_show(),
if the oldformat is detected, the peer_name member of the sockstat
object is left uninitialized (NULL).
For this reason, if a filter has been specified on the command line,
a strcmp() will crash when trying to access it.

Avoid crash by checking that peer_name is not NULL before
passing it to strcmp().

Cc: Stefano Brivio <sbrivio@redhat.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Antonio Quartulli <a@unstable.cc>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>